Oriented coextruded barrier films of polyvinylidene chloride copolymers

ABSTRACT

Disclosed are biaxially oriented, coextruded multilayer barrier films wherein a substantial degree of oxygen impermeability is retained after orientation. The barrier layer of the films comprise a copolymer of vinylidene chloride and an alkyl acrylate, wherein the copolymer has a melting point about equal to or less than the melting point of the thermoplastic material comprising the outer layers of the film and has an oxygen permeability of less than 0.25 cc-mil/100 sq. in.-atm-day based upon the thickness of the barrier layer. Further disclosed are biaxially oriented nonshrinkable multilayer barrier films having barrier layers of a copolymer of vinylidene chloride and from more than 6 to about 10 weight percent methyl acrylate. Further disclosed is a process for making the biaxially oriented, coextruded barrier films.

BACKGROUND OF THE INVENTION

This invention relates to a biaxially oriented, coextruded barrier filmcomprising polyolefin skin or outer layers, a core barrier of vinylidenechloride-acrylate copolymer, and adhesive layers therebetween.

Numerous coextruded case barrier films are known in the art such asSaranex® (Trademark of The Dow Chemical Company). Other barrier filmsare seen in U.S. Pat. Nos. 4,561,920, 3,524,795, 3,579,416, and4,714,638. A typical barrier film comprises one or more barrier layersof a gas or liquid impermeable thermoplastic material, and one or morelayers of a water and/or water vapor impermeable second thermoplasticmaterial. Thermoplastics useful in barrier layers include polyvinylidenechloride and copolymers thereof.

Desired is a coextruded barrier film providing improved strength anddurability, increased modulus, non-extensibility, improved opticalclarity, and improved barrier properties. Such improvements in films ingeneral have been effected by uniaxially or biaxially orienting thebarrier film. Biaxial orientation of a film may be carried outsimultaneously or sequentially. In simultaneous orientation, the film isstretched in two dimensions at the same time. The film may also beoriented simultaneously by stretching in both dimensions but at unequalrates of stretching. Simultaneous biaxial stretching may be effected ina conventional blown film process or in a tentering process. Insequential orientation, the film is stretched in two steps or completelyin one dimension and then completely in the other dimension, and isusually accomplished in a tentering apparatus. Sequential orientationmay also be carried out alternately by stretching in one dimension,stretching in the other dimension, and then again in the originaldimension in an iterative manner for any number of cycles. The extent ofsequential stretching in the two dimensions may be equal or unequal.

Previous attempts at biaxially orienting coextruded cast barrier filmsutilizing polyvinylidene chloride or copolymers of polyvinylidenechloride and polyvinyl chloride in the barrier layer have beenunsuccessful because such orientation deleteriously affected the barrierproperties of film. Biaxial orientation, particularly two-stepsequential biaxial orientation, can deleteriously affect the barrierproperties of the film by altering the crystalline structure of thepolymer or copolymer comprising the barrier layer resulting inmicrovoids therein. Microvoids in the barrier layer substantiallyincrease the degree of oxygen permeability, an undesirable property inmost applications utilizing barrier films.

Two-step sequential biaxial orientation exacerbates the formation ofmicrovoids in a multilayer coextruded barrier film more thansimultaneous biaxial orientation because the crystalline structure ofthe barrier layer is altered twice instead of once.

In commercial and industrial applications, biaxial orientation of filmsis usually accomplished by sequential stretching in an on-line tenteringapparatus. Simultaneous biaxial orientation, though useful in impartingas desirable if not more desirable characteristics to films thansequential biaxial orientation, is not as mechanically adaptable toindustrial processes as sequential orientation. Since sequential biaxialorientation is prevalent in industrial processes for making orientedfilm a particularly desirable oriented, coextruded barrier film wouldsubstantially maintain barrier properties upon sequential orientation.

The creation of microvoids can be avoided or substantially reduced byorienting the multilayer coextruded cast barrier film at a temperatureas close as possible to the melting point of the crystalline orsemi-crystalline polymer or copolymer comprising the barrier layer.Orientation of the cast coextruded film at such a temperature ensuresthat the degree of crystallinity of the polymer or copolymer comprisingthe barrier layer will be minimal, and, thus, that void formation willbe minimized.

Orienting a coextruded, cast barrier film at temperature as close aspossible to that of the melting point of the polymer of copolymercomprising the barrier layer is difficult because the melting point ofmost conventional thermoplastic materials which comprise barrier layersis usually greater than the melting point of the thermoplastic materialswhich conventionally comprise outer or skin layers. The temperature atwhich the orientation occurs must be lower than the melting point of theouter layers of the film because the tentering apparatus which impartsthe desired orientation to the film physically contacts and interfaceswith the film at the outer layers. Processing conditions at thisinterface usually dictate that orientation be carried out at atemperature 10°-30° C. below the melting point of the thermoplasticmaterials comprising the outer layers. If the temperature of orientationis substantially below the melting point of thermoplastic materialscomprising the barrier layer, significant formation of microvoids mayresult due to a substantial degree of crystallization in the barrierlayer at that temperature.

Accordingly, there exists a need for an improved coextruded barrier filmwhich retains a substantial degree of oxygen impermeability upon biaxialorientation. There exists a further need for a coextruded barrier filmwhich retains a substantial degree of oxygen impermeability uponsequential biaxial orientation. There exists a further need for acoextruded barrier film utilizing a barrier layer of polyvinylidenechloride or copolymers thereof which retains a substantial degree ofoxygen impermeability upon sequential biaxial orientation.

BRIEF DESCRIPTION OF THE DRAWING

The novel features of the present invention and the context within whichthey are set will be better understood upon reviewing the followingspecification together with the drawing.

FIG. 1 is a graph showing the effect of sequential orientation upon thedegree of oxygen permeability of different samples of films compared tothe composite cast-film of the corresponding samples.

FIG. 2 is a graph showing the effect of simultaneous orientation uponthe degree of oxygen permeability of different samples of films comparedto the composite cast-film of the corresponding samples.

FIG. 3 is a greatly enlarged isometric view of a biaxially oriented,coextruded barrier film of the present invention.

SUMMARY OF THE INVENTION

A coextruded, sequentially biaxially oriented barrier film according tothe present invention comprises first and second outer layers ofthermoplastic materials and an intermediate barrier layer of a copolymerof vinylidene chloride and an alkyl acrylate. The copolymer has amelting point about equal to or less than the melting point of each ofthe thermoplastic materials of the outer layers. The film further has anoxygen permeability of less than 0.25 cc-mil/100 sq. in.-atm-day at 23°C. based upon the thickness of the barrier layer of theacrylate-vinylidene chloride copolymer. The preferred alkyl acrylate ismethyl acrylate.

A simultaneously biaxially oriented coextruded barrier film according tothe present invention comprises first and second outer layers ofthermoplastic materials and a barrier layer of a copolymer of vinylidenechloride and an alkyl acrylate. The film has an oxygen permeability ofless than 0.13 cc-mil/100 sq. in.-atm-day at 23° C. based upon thethickness of the barrier layer of acrylate-vinylidene chloridecopolymer. The preferred alkyl acrylate is methyl acrylate.

A preferred biaxially oriented coextruded barrier film is substantiallynonshrinkable and comprises a first outer layer of a thermoplasticmaterial, a second outer layer of a thermoplastic material, and abarrier layer of a copolymer of vinylidene chloride and methyl acrylatewherein the copolymer is from about more than 6 to about 10 percent byweight methyl acrylate.

A process for preparing the sequentially or a simultaneously biaxiallyoriented barrier film according to the present invention comprisescoextruding a first and second outer layers of resinous thermoplasticmaterials and an intermediate barrier layer of a resinous copolymer ofvinylidene chloride and an alkyl acrylate to form a composite cast film,decreasing the temperature of the composite cast film below the meltingpoint of the thermoplastic materials of the outer layers, optionallyadjusting the temperature of the composite cast film to within about 10°C. of the melting point of the copolymer and less than the melting pointof the thermoplastic materials of the outer layers, and biaxiallyorienting the composite cast film. Preferably, the copolymer of thebarrier layer has a melting point about equal to or less than themelting point of the thermoplastic materials comprising each of theouter layers.

DETAILED DESCRIPTION

A biaxially oriented, coextruded barrier film according to the presentinvention is referenced generally by the numeral 10, and is illustratedin the single Figure. Film 10 comprises outer layers 26 and 28 and anintermediate barrier layer 20. Preferably, the film 10 further comprisesadhesive layers 22 and 24 situated between outer layers 26 and 28 andbarrier layer 20.

Outerlayers 26 and 28 are comprised of a thermoplastic material such asa polyolefin, a polyester, a polyamide, or a polyvinyl aromatic. Each ofouter layers 26 and 28 may comprise the same or a differentthermoplastic material. A preferred thermoplastic material comprisingouter layers 26 and 28 is polyolefin. Suitable polyolefins includepolyethylene, polypropylene, copolymers of ethylene and propylene, orblends of polyethylene and polypropylene. Suitable polyethylenes includelow density polyethylene, liner low density polyethylene, and ultra lowlinear density polyethylene. The most preferred polyolefin for use inouter layers 26 and 28 is polypropylene.

Adhesive layers 22 and 24 increase the degree of adhesion betweenbarrier layer 20 and outer layers 26 and 28. The resins of polymers orpolymeric compositions suitable for use in adhesive layers are wellknown in the art, and include ethylene-vinyl acetate copolymers andvarious alkyl acrylate polymers such as ethyl acrylate andisobutylacrylate. The preferred material for adhesive layers 22 and 24is an ethylene-vinyl acetate copolymer having about from 10 weightpercent to about 40 weight percent vinyl acetate. The most preferredmaterial for adhesive layers 22 and 24 is an ethylene-vinyl acetatecopolymer having about from 20 weight percent to about 30 weight percentvinyl acetate.

Barrier layer 20 preferably comprises a copolymer of vinylidene chlorideand an alkyl acrylate or alkyl alkacrylate such as methyl acrylate,ethyl acrylate butylacrylate, or methyl methacrylate. The most preferredalkyl acrylate is methyl acrylate. A preferred barrier layer formingcomposition for integration into a sequentially biaxially oriented filmcomprises a copolymer of vinylidene chloride and methyl acrylate whereinmethyl acrylate is present at more than 6 percent to about 10 percent byweight. A preferred barrier layer forming composition for integrationinto a simultaneously biaxially oriented film comprises a copolymer ofvinylidene chloride and methyl acrylate wherein methyl acrylate ispresent from about 6 percent to about 10 percent by weight. Copolymersof vinylidene chloride and methyl acrylate exhibit a lower degree ofcrystallinity at ambient temperature than vinylidene chloridehomopolymers and copolymers of vinylidene chloride and vinyl chloride.This lower degree of crystallinity allows for improved orientationbehavior, and results in a film having better barrier characteristics. Amore preferred barrier layer forming composition comprises as vinylidenechloride-methyl acrylate copolymer wherein the methyl acrylate ispresent in from about 7.5 percent to about 8.7 percent by weight. A mostpreferred barrier layer forming composition comprises a copolymer ofvinylidene chloride and methyl acrylate wherein the methyl acrylate ispresent in about 7.5 percent by weight.

Copolymers of vinylidene chloride and alkyl acrylates, particularlymethyl acrylate, exhibit several desirable properties which make themsuitable for incorporation in barrier layer 20 and superior tohomopolymers of vinylidene chloride and polyvinylidene chloridecopolymers. These properties include lower crystalline melting points,lower rates of crystallization, and lower equilibrium levels ofcrystallinity.

Copolymers of vinylidene chloride and alkyl acrylates, particularlymethyl acrylate at acrylate proportions of about 5 percent or more byweight, have lower melting points than homopolymers of vinylidenechloride or copolymers of vinylidene chloride and vinyl chloride ofequivalent molecular weight. The crystalline melting point of acopolymer of vinylidene chloride and alkyl acrylates decreases as theproportion of acrylate in the copolymer increases. In copolymermolecular weight ranges commonly utilized in film applications, a methylacrylate proportion of more than about 6 weight percent may result in abarrier layer copolymer having a melting point below that of manythermoplastic materials which commonly comprise outer layers, includingpolypropylenes and copolymers of ethylene and propylene whereinpropylene is the major component. If the barrier layer copolymer has amelting point lower than that of the polymer comprising the outer layersand about the same as the desired orientation temperature of saidpolymer, then the cast film may be tentered at or about the meltingpoint of the copolymer. Tentering or orienting at or close to themelting point of the barrier layer copolymer ensures that crystallinitywill be minimal or nonexistent. The lower the degree of crystallinity ofthe barrier layer copolymers upon orientation, the lower the incidenceof void formation, and, thus, the lower the gas permeability.

Copolymers of vinylidene chloride and methyl acrylate further have lowerrates of crystallization and lower equilibrium levels of crystallinitythan copolymers of vinylidene chloride and vinyl chloride. These lowerrates and equilibrium levels also result in improved orientabilitybehavior, which results in lower gas permeability of the oriented film.

Film 10 is prepared according to the present invention by coextruding aplurality of resinous thermoplastic materials to form a composite castfilm of first and second outer layers, an intermediate barrier layer,and adhesive layers therebetween at a temperature at or about themelting point of each of the resinous thermoplastic materials of thelayers, decreasing the temperature of the outer layers of the compositecast film below the melting point of their thermoplastic materials,optionally adjusting the temperature of the composite cast film towithin about 10° C. of the melting point of the copolymer of the barrierlayer and less than the melting point of the thermoplastic material ofmaterials comprising each of the outer layers and, and biaxiallystretching the composite cast film either simultaneously or sequentiallyin two steps. The biaxially oriented film is then further processed asdesired or cooled to ensure the physical integrity of the barrier layer.

Decreasing of the temperature of the outer layers below their meltingpoint after coextrusion is a necessary process step in a casting processbecause the cast composite film is in a molten or semi-molten state uponexiting the coextruder. Decreasing of the temperature of the outerlayers to a submolten level such as by quenching by a chill roll ensuresthe handleability of physical integrity of the outer layers of thecomposite cast film as they subsequently contact various mechanicalsurfaces such as those of a tentering apparatus. Preferably, thetemperature of the entire composite cast film is decreased below themelting point of the thermoplastic materials of the outer layers and thebarrier layers after coextrusion. More preferably, the temperature ofthe entire cast film including the barrier layer is decreased below themelting point of all thermoplastic materials comprising the film. Mostpreferably, the temperature of the entire composite cast film decreasedto an ambient level after coextrusion. Reduction of the temperature ofthe barrier layer after coextrusion is necessary to minimizecrystallinity development within the barrier copolymer. The temperatureof the composite cast film is then adjusted (increased if composite castfilm is at ambient temperature) to the desired orientation temperaturebut below the melting point of the outer layers of the composite castfilm.

If one outer layer is composed of a different thermoplastic materialcomprising the other outer layer, the temperature of the outerlayersmust be decreased after coextrusion to below the melting point of thethermoplastic material having the lower melting point of the two tofacilitate subsequent handling of the film.

Biaxial orientation of the cast composite film forming oriented film 10may be carried out simultaneously or sequentially. A typical castcomposite film may be stretched from about one to fifty times itsoriginal dimensions. More typically, cast composite films will bestretched from about two to twenty times its original dimensions. Mosttypically, a cast composite film will be stretched from about three toabout ten times its original dimensions.

As film resin extrudate exits the die in a blown or casting filmprocess, a minor degree of uniaxial orientation is imparted to theextrudate. This impartation is negligible in view of the much moresubstantial biaxial orientation which occurs in the tentering apparatusor the bubble.

Preferably, films according to the present invention will besubstantially nonshrinkable. A film is substantially nonshrinkable whenabout 10 percent or less shrinkage occurs when the the oriented film isimmersed in boiling water. Nonshrinkability may be imparted to theoriented film by heat setting of the film.

The lower the degree of crystallinity of the barrier layer at the timeand temperature the film is oriented, the lower the incidence of theformation of microvoids. The lower the incidence of microvoids in theoriented film, the lower the degree of oxygen permeability. It isfurther desirable to conduct such orientation at or about the glasstransition temperature of the thermoplastic materials comprising theouter layers.

The temperature at which the orientation of the cast composite filmforming oriented film 10 is carried out is determined by thecharacteristics of the thermoplastic materials comprising both thebarrier and outer layers.

For thermoplastic outer layers 26 and 28, the temperature of orientationis preferably between their glass transition temperatures and theirmelting points to ensure the impartation of the desired orientation tothe crystalline or semi-crystalline structure of the material. For mostthermoplastic materials commonly comprising the outer layers such as thepolyolefins, the desirable orientation temperature ranges from about 10to about 30° C. below the melting point of the material. Forpolypropylene, the most preferred orientation temperature is about 20°C. below its melting point, and for low liner density polyethylene, themost preferred orientation temperature is about 10° C. below.

For barrier layer 20, the temperature of orientation is preferablyhigher than the glass transition temperature of and within about 10° C.above or below the melting point of the thermoplastic materialcomprising the same. Upon orientation of barrier layer 20 at suchtemperature, optimum barrier properties, most notably a substantialdegree of oxygen impermeability, are maintained. Most preferably, thetemperature of orientation is at about the melting point of thethermoplastic material comprising barrier layer 20.

Film 10 of the present invention is especially adapted to maintain asubstantial degree of gas, including oxygen, impermeabilty aftersequential or simultaneous biaxial tentering. A sequentially biaxiallyoriented film 10 will have an oxygen permeability of preferably lessthan 0.25, more preferably less than 0.13, and most preferably less than0.09 cc-mil/100 sq.in./atm-day at 23° C. based upon the thickness ofbarrier layer 28. A simultaneously biaxially oriented film 10 will havean oxygen permeability of preferably less than 0.13, more preferablyless than 0.09, and most preferably less than 0.06 cc-mil/100sq.in/atm-day at 23° C. based upon the thickness of barrier layer 20.

A most preferred sequentially biaxially oriented film comprises outerlayers of polypropylene, a barrier layer of a copolymer of vinylidenechloride methyl acrylate wherein the methyl acrylate comonomer ispresent at from more than 6 percent to about 10 percent by weight, andadhesive layers situated between the barrier layer and the outer layers.The adhesive layers preferably comprise an ethylene-vinyl acetatecopolymer having a vinyl acetate content of between 20 and 30 percent byweight.

The examples below are provided for purposes of illustration, and aremot to be construed as limiting. All percentages are by weight unlessindicated otherwise. All oxygen permeabilities are based upon thethickness of the barrier layer.

EXAMPLES 1 and 2

The oxygen permeabilities of both sequential and simultaneous biaxiallyoriented films and composite cast films of corresponding thicknesseswere measured and compared for the two film types at various orientationtemperatures.

A number of 5-layer composite cast films were coextruded withpropylene-ethylene copolymer outer layers, ethylene-vinyl acetatecopolymer (EVA) adhesive layers, and various SARAN® (trademark of TheDow Chemical Co.) copolymers in the barrier core layers. Hercules 7531propylene-ethylene copolymer was selected for the outer layers in orderto allow orientation of the composite cast films over a wide temperaturerange. The barrier layer in each of the test films was about 12-13percent of the entire film by volume, and each of the adhesive layerswere about 10 percent each by volume. The thickness of both thecomposite cast test films and the oriented test films utilized in theExamples was about 2 mil.

The oriented films were formed by sequential and simultaneous tenteringof 18 mil composite cast films. Tentering was carried out in a T. M.Long Stretching frame in a 3× stretch (three times the originaldimensions) in two dimensions over a temperature range of 100° to 145°C. After the films were extruded, they were quenched and maintained atsubambient temperatures (-8° C.) in order to minimize crystallinitydevelopment of the vinylidene chloride copolymers in the barrier layers.Minimization of crystallinity development is important for purposes ofsimulation of an on-line manufacturing process. In an on-line process,orientation of the film takes place immediately or soon after extrusionand forming of the film. For the oriented test films however, suchorientation could not be effected immediately after extrusion because ofprocessing equipment limitations; therefore, the degree of crystallinityof the freshly extruded composite cast film was maintained by freezinguntil such time as the film could be oriented.

The 2 mil composite cast films were coextruded and not subsequentlyoriented. They were of equivalent thickness to the 2 mil oriented films.

In Sample 1, the barrier layer composed SARAN X0S5253.34, B2000 lowconversion (80/20 vinylidene chloride/vinyl chloride copolymer). InSample 2, the barrier layer composed 6 percent MA SARAN XU32024.00 (94/6vinylidene chloride/methyl acrylate copolymer). In Sample 3, the barrierlayer composed 7.5 percent MA SARAN XU 32023.01 (92.5/7.5 vinylidenechloride/methyl acrylate copolymer). In Sample 4, the barrier layercomposed 8.3 percent MA SARAN XU32025.01 (91.7/8.3 vinylidenechloride/methyl acrylate copolymer). In Sample 5, the barrier layercomposed 8.7 percent MA SARAN XU 32028.01 (91.3/8.7 vinylidenechloride/methyl acrylate copolymer). All of the above percentages andproportions are directed to the balance of the composition of thebarrier layers apart from the above mentioned extrusion aid.

The oxygen permeability of both the composite cast and the orientedfilms was measured on an Ox-Tran 1050 at 23° C. and the data summarizedin Table 1. Although the oxygen permeability for the films as a wholewas measured, the contribution of the outer layers to the permeabilityof the film was considered to be negligible.

The data of Table 1 corresponds to FIG. 1, and the data of Table 2corresponds to FIG. 2.

                                      TABLE 1                                     __________________________________________________________________________    Oxygen Permeability                                                           3 × 3 Sequential Biax                                                   Sample                                                                             % Acrylate                        Cast                                   No.  (by weight)                                                                          100° C.                                                                      115° C.                                                                      130° C.                                                                     138° C.                                                                     145° C.                                                                     Film                                   __________________________________________________________________________    1    0      >1.3  0.79  0.392                                                                              1.25 1.25 0.227                                  2    6      --    ˜1.3                                                                          0.132                                                                              1.0  1.25 0.082                                  3    7.5    --    0.165 0.089                                                                              0.072                                                                              0.096                                                                              0.091                                  4    8.3    0.155 0.131 0.10 0.076                                                                              0.184                                                                              0.107                                  5    8.7    0.199 0.205 0.211                                                                              0.085                                                                              0.156                                                                              0.106                                  __________________________________________________________________________     (cc.mil/100 sq.in. atm. day at 23° C. ± 1° C.)          

In Table 1 and FIG. 1, the data of Sample 1 indicate the negative effectof sequential orientation upon the degree of oxygen permeabilitycompared to the composite cast film of the same sample regardless oftemperature of orientation. The data of Sample 2 indicate a similarnegative effect upon oxygen permeability due possibly to lack ofsufficient depression of the melting point of the copolymer comprisingthe barrier layer. The data of Samples 3-5 indicate an actual decreasein degree of oxygen permeability versus that of the cast film in the130°-140° C. temperature range particularly around the 138° C. point.

                                      TABLE 2                                     __________________________________________________________________________    Oxygen Permeability                                                           3 × 3 Simultaneous Biax                                                 Sample                                                                             % Acrylate                      Cast                                     No.  (by weight)                                                                          100° C.                                                                     115° C.                                                                     130° C.                                                                     138° C.                                                                     145° C.                                                                     Film                                     __________________________________________________________________________    1    0       0.421                                                                             0.356                                                                              0.323                                                                              0.149                                                                              0.979                                                                              0.228                                    2    6      --   0.192                                                                              0.145                                                                              0.050                                                                              0.12 0.082                                    3    7.5    --   0.107                                                                              0.11 0.049                                                                              0.084                                                                              0.091                                    4    8.3    0.11 0.099                                                                              0.148                                                                              0.056                                                                              0.095                                                                              0.107                                    5    8.7    0.12 0.12 0.208                                                                              0.076                                                                              0.099                                                                              0.106                                    __________________________________________________________________________

In Table 2 and FIG. 2, the data of Sample 1 indicate the negative effectof simultaneous orientation upon the degree of oxygen permeabilitycompared to the composite cast film of the same sample regardless oftemperature of orientation. The data of Samples 2-5 indicate an actualdecrease in degree of oxygen permeability versus that of the cast filmin the 130°-140° C. temperature range particularly around the 138° C.level.

EXAMPLE 3

The oxygen permeability of a sequentially biaxially oriented coextrudedbarrier film having polypropylene outer layers was measured. The fivelayer structure is the same as the films found in Examples 1 and 2except that it was 35 mils thick prior to orientation. The barrier layercomprised 8.25 percent MA SARAN XU 32022.00 (91.75/8.25 vinylidenechloride/methyl acrylate copolymer). The cast film was sequentiallystretched 4.5 to 5.5 times in the machine direction and 7 to 9 times inthe transverse direction at 145° C. The oxygen permeability was 0.13cc-mil/100 sq. in.-atm-day at 23° C. based upon the thickness of thebarrier layer.

EXAMPLE 4

The oxygen permeability of a sequentially biaxially oriented coextrudedbarrier film having outer layers of low linear density polyethylene wasmeasured. The five layer structure is the same as the films found inExample 1 and 2 except that it was 35 mils thick prior to orientation.The barrier layer comprised 8.25 percent MA SARAN XU 32022.00(91.75/8.25 vinylidene chloride/methyl acrylate copolymer). The castfilm was sequentially stretched 4.5 to 5.5 times in the machinedirection and 7 to 9 times in the transverse direction at 110° C. Theoxygen permeability was 0.22 cc-mil/100 sq. in.-atm-day at 23° C. basedupon the thickness of the barrier layer.

While the preferred embodiments of the biaxially oriented coextrudedfilm have been shown with regard to specific details, it will beappreciated that depending upon the manufacturing process and themanufacturer's desires, the present invention may be modified by variouschanges while still being fairly within the scope of the novel teachingsand principles herein set forth.

What is claimed is:
 1. A coextruded barrier film, the film beingsequentially biaxially oriented by tentering process, comprising:a) afirst outer layer of a thermoplastic material selected from the groupconsisting of polyolefin, polyester, polyamide or a polyvinyl aromatic;b) a second outer layer of a thermoplastic material selected from thegroup consisting of polyolefin, polyester, polyamide or a polyvinylaromatic; and c) a barrier layer of a copolymer of vinylidene chlorideand an alkyl acrylate, the film having an oxygen permeability of lessthan 0.25 cc-mil/100 sq. in-atm-day based upon the thickness of thebarrier layer.
 2. The film of claim 1, wherein the copolymer has amelting point about equal to or less than the melting point of each ofthe thermoplastic materials of the outer layers.
 3. The film of claim 2,wherein the melting point of the copolymer is from about 10 to about 30degrees C. less than the melting point of each of the thermoplasticmaterials of the outer layers.
 4. The film of claim 1, wherein thethermoplastic materials of the first and second outer layers arepolypropylene.
 5. The film of claim 1, wherein the alkyl acrylate is amethyl acrylate.
 6. The film of claim 5, wherein the copolymer is morethan about 6 percent to about 10 percent by weight methyl acrylate. 7.The film of claim 1, wherein the oxygen permeability is less than 0.13cc.-mil/100 sq. in.-atm-day based upon the thickness of the barrierlayer.
 8. The film of claim 7, wherein the oxygen permeability is lessthan 0.09 cc-mil/100 sq. in.-atm-day based upon the thickness of thebarrier layer.
 9. A coextruded barrier film, the film beingsimultaneously biaxially oriented by tentering, comprising:a) a firstouter layer of a thermoplastic material selected from the groupconsisting of polyolefin, polyester, polyamide or a polyvinyl aromatic;b) a second outer layer of a thermoplastic material selected from thegroup consisting of polyolefin, polyester, polyamide or a polyvinylaromatic; and c) a barrier layer of a copolymer of vinylidene chlorideand an alkyl acrylate, the copolymer having a melting point about equalto or less than the melting point of each of the thermoplastic materialsof the outer layers, the film having an oxygen permeability of less than0.13 cc-mil/100 sq. in.-atm-day based upon the thickness of the barrierlayer.
 10. The film of claim 9, wherein the copolymer has a meltingpoint about equal to or less than the melting point of each of thethermoplastic materials of the outer layers.
 11. The film of claim 10,wherein the melting point of the copolymer is from about 10° to about30° C. less than melting point of each of the thermoplastic materials ofthe outer layers.
 12. The film of claim 9, wherein the thermoplasticmaterials of the first and second outer layers are polypropylene. 13.The film of claim 9, wherein the alkyl acrylate is a methyl acrylate.14. The film of claim 13, wherein the copolymer is more than 6 to about10 percent by weight methyl acrylate.
 15. The film according to claim 9,wherein the film has oxygen permeability of less than 0.09 cc-mil/100sq. in.-atm-day.
 16. The film according to claim 15, wherein the filmhas oxygen permeability of less than 0.06 cc-mil/100 sq. in.-atm-day.17. A coextruded barrier film, the film being sequentially biaxiallyoriented by tentering and substantially nonshrinkable, comprising:a) afirst outer layer of a thermoplastic material selected from the groupconsisting of polyolefin, polyester, polyamide or a polyvinyl aromatic;b) a second outer layer of a thermoplastic material selected from thegroup consisting of polyolefin, polyester, polyamide or a polyvinylaromatic; and c) a barrier layer of a copolymer of vinylidene chlorideand methyl acrylate, the copolymer being from about more than 6 to about10 percent by weight methyl acrylate.
 18. A coextruded barrier film, thefilm being simultaneously biaxially oriented by tentering andsubstantially nonshrinkable, comprising:a) a first outer layer of athermoplastic material selected from the group consisting of polyolefin,polyester, polyamide or a polyvinyl aromatic; b) a second outer layer ofa thermoplastic material selected from the group consisting ofpolyolefin, polyester, polyamide or a polyvinyl aromatic; and c) abarrier layer of a copolymer of vinylidene chloride and methyl acrylate,the copolymer being from about more than 6 to about 10 percent by weightmethyl acrylate.